In a process of manufacturing a semiconductor device, tungsten is used as material for filling contact holes and via holes between wirings, material for forming wirings and diffusion barriers, and the like, which are all formed in a semiconductor wafer (hereinafter, simply referred to as a “wafer”) as a target object.
Physical vapor deposition (PVD) has been used in tungsten film deposition in the art. However, tungsten (W) is a metal having a high melting point, and it is difficult to achieve high step coverage corresponding to a recent trend of miniaturization of devices using PVD. For this reason, a tungsten film is formed using chemical vapor deposition (CVD), which does not require melting of tungsten having a high melting point and is capable of achieving sufficient miniaturization of devices.
In a tungsten film forming method using CVD (CVD-tungsten film), for example, tungsten hexafluoride (WF6) as a source gas and H2 gas as a reducing gas are generally used to cause reaction of WF6+3H2→W+6HF on a wafer. Also, in recent years, atomic layer deposition (ALD), in which WF6 gas and the reducing gas are alternately supplied, has attracted attention as a technology for providing higher step coverage.
In the tungsten film formation through CVD or ALD using WF6 as a source gas, it is difficult to form a good tungsten film on a base film such as a TiN film. Accordingly, a two-step film formation process is performed wherein an initial tungsten film (new creation film) for nucleus generation is first formed and a main tungsten film is formed on the new creation film.
However, the new creation film is formed for nucleus generation and has a higher resistance than the main tungsten film in a bulk state. Recently, as semiconductor devices are further miniaturized, the ratio of the new creation film in the tungsten film buried in a depression is also increased. Therefore, it is likely that the new creation film having a high resistance deteriorates the overall resistance of the tungsten film. Moreover, formation of the new creation film in addition to the main tungsten film requires a complicated process.